Aqueous dispersions of cellulose-reactive sizing agents, such as, for example, alkenyl succinic anhydride ("ASA") and alkyl ketene dimer ("AKD"), are typically used for sizing paper products to impart hydrophobic properties to the paper. The sizing agents arc introduced during the paper making process at the wet end, and, thus, small sized particles of the sizing agents must be uniformly dispersed throughout the fiber slurry to maximize the sizing effect. Typically, improved emulsification performance of sizing agents, is judged by the median particle size of the emulsion, as a smaller particle provides better sizing and is more efficient, requiring less material, which lowers cost. Therefore, sizing agents are typically added in the form of an aqueous emulsion that includes an emulsifying agent that reduces agglomeration and separation. Typical emulsifying agents include, for example, cationic and ordinary starches, carboxymethyl cellulose, natural gums, gelatin, cationic polymers, and vinyl alcohol. The use of such emulsifiers, which may be used with or without surfactants, requires the use of relatively complex equipment that is capable of exerting at least one of high shear and high pressure to provide the homogenization required to form the desired emulsion.
ASA in particular is very widely used sizing agent in alkaline fine paper, but is very reactive, and not chemically or physically stable when stored for an extensive period of time. It must therefore be emulsified on-site with a cationic emulsifier.
Paper mills equipped with on-site cooking equipment for cooking starch typically use a starch that is shipped as a dry solid, which is cooked on-site, diluted, and mixed with ASA just prior to being introduced into the paper making process as a stable emulsion. Thus, in such mills, ASA is typically emulsified immediately before use, and fed directly to the paper machine. Cationic starches that are cooked on site have been used very successfully to emulsify ASA, stabilize the ASA emulsion particles, and help to retain them on negatively charged cellulose fibers and other furnish components. As the use of ASA is expanded in board mills, which produce such products as linerboard, gypsum board, white top board, and are typically not equipped with starch cookers, the availability of a ready-to-use emulsifier for ASA is necessary, as ASA emulsions are unstable over time, and will separate and hydrolyze if formed and shipped from off-site. Such plants require a pumpable liquid cationic emulsifying and stabilizing agent that can be mixed with ASA on-site without cooking. In addition, even mills other than board mills can benefit from a ready-to-use starch emulsion that is in liquid form.
There are relatively few liquid starches commercially available. One such cationic liquid starch is REDISIZE 132, available from National Starch, but it is not as efficient as would be desirable, and, thus, larger amounts are needed to provide sufficient sizing. While high solids of the liquid starch is desirable to minimize shipping costs, once the starch is diluted for emulsification, it is then desirable to use as little starch as possible. However, such starches are often not suitable for ASA emulsification at lower solids, i.e., below about 7%. As a result, such starches are used at higher solids, which requires higher starch consumption and higher costs to the customer. Therefore, a need exists for a liquid starch that can provide a good quality emulsion of ASA and other sizing agents at lower starch solids, i.e., below about 7%, which would provide a low starch to size ratio and a small particle size emulsion, and provide better and/or more efficient sizing.
Moreover, where cooked starches and hydrocolloids are used, in addition to the elevated temperatures required for cooking, high shear and/or high pressure are required to form the emulsion. As a result of the complicated procedures and equipment required, problems with the deposition of the sizing emulsion in the paper system, quality control, and generally unsatisfactory results often occur.
Various attempts have been made to improve sizing emulsions. For example, U.S. Pat. Nos. 4,214,948 and RE 29,960 to Mazzarrilla, et al. disclose the internal sizing of paper with a sizing emulsion consisting essentially of a substituted cyclic dicarboxylic acid anhydride or a higher organic ketene dimer and a polyoxyalkylene alkyl or polyoxyalkylene alkyl-aryl ether or the corresponding mono- or diester.
U.S. Pat. No. 5,876,562 to Johansson discloses a sizing dispersion of a cellulose-reactive sizing agent and colloidal anionic aluminum modified silica-based particles.
U.S. Pat. No. 5,595,631 to Tsai et al. discloses a method of sizing paper products using a size composition comprising an aqueous dispersion of a cyclic dicarboxylic acid anhydride containing hydrophobic substitution and a dry, cationic, non-degraded starch that is modified with either an ether group or an ester group.
However, a need remains in the art for sizing emulsions having improved emulsion quality, sizing performance, and operability, which do not require cooking at the mill. The present invention provides such a sizing emulsion. The teachings of all patents and other literature articles referenced herein are incorporated herein by reference to the extent necessary to supplement this specification.